259125 Phase Transition of Ceria Using First Principles Calculations
Ceria can undergo phase transformation from its stable stoichiometric ratio at ambient conditions to a lower oxygen content material with increasing temperature and decreasing oxygen partial pressure, while preserving its fluorite structure (Fm-3m). However at a critical O/Ce ratio of 1.5, the hexagonal structure (P3/m1) of ceria becomes more prevalent.1,2 The transition from a stoichiometric to non-stoichiometric ceria creates oxygen vacancies along with cerium atoms with a reduced oxidation state.3,4 Such characteristics make ceria a promising material for catalytic reactions.1,5 One such case is the application of ceria in the Water-Gas Shift (WGS) reaction, where metals supported on ceria exhibit higher activities compared to no ceria support or a different support material.6,7,8 Our work probes an atomic level understanding of the nature of ceria over a wide range of temperature and pressure conditions using Density Functional Theory (DFT) and First Principle Thermodynamics (FPT). The most stable ceria (111) surface9,10 is modeled using a periodic 2x2 supercell image, as shown in Figure 1. More than 25 configurations of oxygen concentrations are considered to determine the stable phases in a wide temperature pressure region. This is the first ever comprehensive first principles investigation to predict the phase transition of ceria under various operating conditions.
Left: Top view and Right: Side view of the ceria (111) plane. Blue: adsorbed oxygen; red: surface oxygen; black: sub-surface oxygen; green: surface cerium; and grey: cerium in the second layer.
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